Option variable
current_let_rule_packageDefault value: default_let_rule_package
current_let_rule_package is the name of the rule package that is used by
functions in the let package (letsimp, etc.)
if no other rule package is specified.
This variable may be assigned the name of any rule package defined
via the let command.
If a call such as letsimp (expr, rule_pkg_name) is made,
the rule package rule_pkg_name is used for that function call only,
and the value of current_let_rule_package is not changed.